In many gas turbine engines, efficiency is promoted by minimizing running clearances between seals on a case of the engine and the blade tips of compressor blades and turbine blades on a gasifier rotor of the engine. One way such clearances are minimized is by rigorous control of the assembled dimensions of the gasifier rotor. For example, in a gasifier rotor where the turbine is mounted on one or more mounting pilots of a gasifier shaft bolted to the rotating group of a compressor, manufacturing tolerance stack-up may render it necessary to custom grind the mounting pilots to achieve required assembled dimensions. A gas turbine engine rotor according to this invention incorporates an adjustable shim pack which obviates the need for custom grinding of the mounting pilots.